Modulation system



' 1942. R. E. TAYLOR MODULATION SYSTEM Filed May 20, 1940 2 Sheets-Sheet1 I|= .;L

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MODULATION n INVENTOR.

. KW am y Patented May 12, 1942 UNITED STATES PATENT OFFICE I 2,282,341MODULATION SYSTEM Robert E. Taylor, LosAngeles, Calif. Application May20, 1940, Serial No. 336,256 14 Claims. (01. Ive-171.5)

My. invention relates broadly to modulation systems and moreparticularly to a system for simultaneous amplification and modulationat high efiiciency.

One of the objects of my invention is to provide means for modulating ahigh frequency carrier wave at high efliciency while at the same timeamplifying the carrier at high efliciency for delivering a signal waveof relatively high power for a given input with a minimum of equipment.The system, therefore, is adapted particularly for mobile transmitters,such as p0- lice, relay and aircraft, as well as for televisiontransmission and commercial high power systems where great amplificationand high efficiency are required.

Another object of my invention is to provide a system for amplificationand modulation of a high frequency carrier wave at high efiiciency bymeans of separate circuits biased to cut-off at different currentlevels, whereby positive modulation peaks are delivered from one circuitand negative modulation peaks from another circuit, both circuitsoperating at high efiiciency.

A further object of my invention is to provide a modulation andamplification system including a separate circuit for delivering thepositive modulation peaks, whereby the basic modulation circuit may beoperated at high efilciency, and an output circuit for combining thebasic modulated wave and the peaks in proper phase relation.

Still another object of my invention is to provide a simplified highefiiciency amplification system for voltages of carrier frequencieswherein different portions of the wave are amplified in differentcircuits and combined in the output, the system being operative also formodulating the carrier wave.

A still further object of my invention is to provide a high frequencyamplification system wherein different portions of the wave areamplified in different circuits for high efiiciency and combined in theoutput, the amplification sys tem being operated also to modulate thehigh frequency wave being amplified to provide low frequency componentsin the different circuits, whereby combination of the different portionsof the high frequency wave may be effected in the output circuit inproper phase by adjustment with reference to the low frequencymodulation in simplified manner.

The invention, therefore, should not be confused with customary higheificiency amplifiers, as for instance the Doherty Circuit, foramplification of modulated waves which are essentially of highfrequency, and wherein high frequency phase relations must be accuratelycontrolled if the peak portions are to synchronize with the baseportions of the wave'in the output. The input as well as the outputbranches of the Doherty system require unusual skill for theiradjustment, besides requiring complicated networks, as is well known inthe art.

In the system of my invention, both modulating voltages and carriervoltages are applied independently to appropriate simple input networksand the modulated and amplified components are superimposed in a simplestraightforward output branch of the system. For this reason highfrequency phase relations are not critical and the adjustment of thenovel high efilciency system is simple.

The invention will be more clearly understood from the followingdescription of practical embodiments of the invention made withreference to the accompanying drawings in which:

Figure 1 is a schematic diagram of the/circuit arrangement ofmy-invention showing the principal elements thereof in cooperativerelation; Fig. 2 is a schematic diagram of a more complete arrangementemploying basically the same elements shown in Fig. 1; Fig. 3 is aschematic diagram of a modified form of my invention; and Fig. 4 is atheoretical graphical representation of the modulated wave components assuperimposed in the output circuit.

Referring to Fig. 1 which primarily illustrates the principles of myinvention, electron tubes I and 2 are operative simultaneously asamplifiers and modulators, tube I being employed as a low level class Camplifier to deliver modulated high frequency current up to apredetermined maxi- .mum at high efliciency, while tube 2 is employed asa high level class C amplifier to deliver the peak portions of themodulated wave beyond the maximum set for.tube l and to operate likewiseat high efliciency. Referring to Fig. 4, the output current of tube I isshown at 11 with a relatively fiat characteristic at the predetermined 420 and I9, respectively, as will bemore fully described.

Tube I includes anode 3,; control grid 4 and cathode 5, while tube 2includes anode 8, control grid 1 and cathode 8. Anode 3 obtainsoperating potential from a source at 31 through choke '9, while themodulated output current 11 passes blocking condenser .II and throughinductance I6 and by-pass condenser I3 to ground; cathode I 5 isgrounded through meter ll'which measures the output current I1. 'Anode 8connects with inductance I6 at tap II, which is substantially central ofthe inductance, and delivers the out-v output current I2.. In the lowerportion of in-- ductance I6, below tap II, the currents I1 and I2 arecombined substantially as represented in Fig. 4. The modulated output isapplied to a load represented by resistance I8 through inductance 45coupled with inductance I8. Tuning condenser I2 is providedsubstantially in shunt with inductance It for tuning the output circuitto resonance at the common high frequency of the carrier componentsdelivered fromtubes I and 2.

I through a coupling link 40 from any suitable source, not shown. Atuned circuit including inductance 4| and condenser 43, and blockingcondenser 42, is coupled with the link 40 and has a connection directlyto control grid l of tube 2 and a branch connection through couplingcondenser 23 with the control grid 4 of tube I, whereby carrier energyis supplied to tubes Zand l for amplification. A tuned circuit includininductance 25 and condenser 24, and blocking condenser 26, is connectedbetween grid 4 and cathode 5 of tube i, in resonance with the carrierenergy for stabilizing the operation. pling condenser 23 may bevariable, as shown, for adjusting the carrier drive in the respectivegrid branches of tubes I and 2 for maximum over-all emciency. The tubes5 and 2 individually operate as class C amplifiers at high efliciency byvirtue of the high negative grid biases from sources and i9,respectively.

condenser 29, high frequency choke 21 and in-' ductance 25 to grid 4 oftube I; and similarly,

- low frequency energy from both inductances and Si is applied throughhigh frequency tuned inductance dl to grid I of tube 2. Bias source 20connects from ground through resistance 28, choke 21' and inductance 25to grid 4, while bias source i9 connects from ground through resistance33, which shunts inductance 3i, and through inductances 30 and 6| togrid I. Resistance 28 is in circuit with secondary induct-. ance 3I anda portion of the bias on grid 4 is due to the voltage drop acrossresistance -28 due to the low. frequency current and the rectified highfrequency current. Similarly, low frequency current in resistance 33from secondary inductance 3| produces a voltage drop effective for thebias on grid I. At the same time, resistance 33 and condenser 29 are soconstituted as to proportion the amount of low frequency modulationCouenergy supplied to tubes I and 2; tube I has impressed thereon a lowfrequency voltage the positive excursions of which are limited, whiletube 2 receives the positive peaks of the low frequency 5 voltage whichrequire the greater proportion of the low frequency power for effectiveoperation. The relative power outputs of the two tubes, however, aredependent upon other factors as well as the proportion of low frequencyinput, and are'adjusted for'maximum over-all efliciency in the system.

' The grid voltage in tube I is limited by the action of resistance 2|and condenser 22, series connected in shunt between grid 4 and cathode5. Resistance 2I limits the carrier frequency voltage and regulates thelow frequency peaks, while 22 acts as a blocking condenser to maintainthe bias potential of grid 4.

Many experiments show that the system of my invention is admirablyapplicable to short wave systems, and that there is no difficulty atallin adjusting the circuits to operate at high efficiency and with goodfidelity even at 30 megacycles and higher. This order of magnitude-offrequency is given only as an illustration however, and not as a limitas regards the carrier frequency. The condenser 23 does not have to bevariable, although by means of a suitable var.-

iation the magnitude of the energy impressed On tube I can beestablished at the best value. This variation is by no means criticaland after having found the appropriate setting, a fixedcondenser can beused. It is to be understood that in my system condenser 23 is not usedfor pro ducing a ninety degree phase shift since I do not use impedanceinverters in my system, as is, for instance, the case in the Dohertysystem. The carrier frequency reactance of condenser 23 is not requiredto have a certain impedance rela tion to resistance 2|, since in mysystem this resistance merely relieves grid 4 of excessive controlvoltage at times when the applied voltage lies within the most positivecap portion of the low frequency wave.

.It can therefore be seen, so far as the input elements of my system areconcerned, that there can be no great difliculty in the adjustment ofthe elements. From the simplicity of the dynamic plate branches it isalso seen that only tuning by means of condenser I2 is necessary. Thesimplicity of the system of my invention may perhaps be best appreciatedby describing how the circuits are adjusted.

With' tube 2 removed, the negative grid bias 20 I of tube i is set atabout twice the cut-off voltage foran appropriate steady plate potentialapplied to anode 3 through high frequency choke 9. The tuning condensers24 and .12 in the input and output branches of tube I are varied untilmaximum power is noted in the load branch 45, that is, until maximumcurrent from tube iflows over the proper resistance I8. The currentmeasured steady supply voltage on anode 6 applied through high frequencychoke I 0 and tap I7 on coil I5. The input branch of tube 2 is thentuned by "0 means of condenser 43 and condensers 24 and ii? are slightlyretuned until absolute resonance in the input and output is established.Under these conditions, there is essentially the same power transfer tothe load branch45 since for such a high negative bias I9 on grid 1, tube2 is blocked,

that is, no current flows through meter I and therefore no power isdelivered from the anode circuit of tube 2. The negative grid bias I9 isnow decreased until a small plate current begins to flow in tube 2. Inthe same experiment meter I5 registered 10 milliampers. With samecarrier power excitation acting on coil 4I and no modulation powerapplied through coil 44, the settings of condensers 24 and 43 areadjusted until maximum response is secured in tube 2; this condition isreached when the meter I5 indicates a maximum current, This adjustmentdoes not put the circuits oil resonance as condensers 24 and 43 aredynamically in multiple; the adjustment gives, however, a change intheL/C ratio without a change of the CL product. In the'experiment heredescribed, the current maximum on meter I5 was milliamperes. Thenegative bias I9 is then again increased until cut-off occurs, that is,until the meter I5 again indicates zero current. In addition, themodulation voltage is now applied by means of excitation in output coil44 and increased until maximum power is obtained in the load branch 45,that is, until maximum current flows in resistance I8; this maximum willbe about twice that obtained from tube I alone. For 100% modulation thereading of meter I5 increased to about 60 milliamperes and the readingin meter I4 decreased to about milliamperes.

It is to be understood that internal modulation can be accomplished alsoby cathode modulation, and in screen grid tubes by applying themodulation energy to the screen. I have found, also that the beam tubesare especially suitable. Neutralization is avoided by using the screengrids of such tubes for preventing objectionable feedback from anode tocontrol grid.

Fig. 2 schematically shows the use of beam tubes in the positions oftubes I and 2 of Fig. 1,,

with the major portion of the circuit of Fig. 2 the same as in Fig. 1;like reference characters indicate like elements in Fig. 2 so that itwill be necessary only to describe the modifications introduced in Fig.2. Elements 5| in tube I and 53 in tube 2 represent the beam formingelectrodes connected with the respective cathodes 5 and 8. Screen gridsare shown at 52 in tube I and at 54 in tube 2, and are energized throughseries resistors 41 and 48, respectively from the source of anodepotential 39. Conventional bypass condensers are provided at 45 and 46.Meter in the negative bias circuit for grid, I of tube 2, meter 36 inthe negative bias circuit'for grid 4 of tube I, plate current meter I5of tube 2, plate current meter I4 of tube I, and meter 38 of the loadbranch are used for adjusting to optimum output power. The adjustment iscarried on as described above in connection with Fig. 1. The pick-upcoil in the load branch connects to ground and over a high frequencyammeter 38 and through a concentric line 49 to aerial 50.

A single source of anode potential is provided at- 39 to supply theanode power to both tubes I and 2 instead of the separate sources ofFig. 1. Choke coils 9 and I0 and blocking condenser II are effective asin Fig. 1, however. for isolating the separate anode potentials whileby-pass condenser I3 provides a path for the high frequency currents toground, as in Fig. 1.- Various' other arrangements may be employed forapplying the proper anode potentials, from one or separate sources, tothe anodes 3 and 6. Resistance 33 isomitted and choke 34 is added in thelow frequency supply circuit to tube 2 as a further varia- Y 3 tion fromthe circuit of Fig. 1. Also. resistance ztis connected from the grid toa point above blocking condenser 26, thus permitting elemination ofblocking condenser 22 in Fig. 2.

It is to be understood that the modulation power can also be supplied bymeans of a. choke and condenser couplingarrangement from the modulationinput orby using a peaking coil in series with a suitable resistor inthe modulation input coupling condenser 23a, tap 55 and transferring thepower either directly or over a condenser or other suitable coupling tothe separate tubes I and 2. Parallel peaking circuits can also be used.These various arrangements are applicable especially where the system ofmy invention is used in connection with television transmission. It isto be further understood that ordinary screen grid tubes can be usedalthough beam tubes, as indicated in Figs. 2 and 3, are especiallyeffective in the upper megacycle range.

Fig. 3 shows another modification of the novel system of myinvention.-Tubes I and 2 are of the type shown in Fig. 2, with anode potentialssupplied from the source 39 through choke I0 and load inductance I6 toboth anodes 3 and 6, anode 6 being connected at tap IT as in Fig. 1 andblocking condenser II being eliminated. The output is supplied toantenna 50 from a tap connection on load inductance It in lieu ofpick-up coil 45. The particular feature of Fig. 3 resides in theprovision of only one tuning condenser 43 in the carrier input circuitwhile there are two tuning condensers I2 and provided in the respectiveanode circuits of tubes I and 2. Tap 55 on input coil 4| and condenser23 are used for adjusting the carrier input to tubes I and 2; tube Ireceives carrier energy through and the portion 4Ib of coil 4I, whiletube 2 receives carrier energy direct from coil 4| by connection to theupper portion 4Ia. Limiting resistor 2I and blocking condenser 22 areprovided for tube I as in Fig. l. The low frequency modulation inputissubstantially the same as in Fig. 2, the grid I of tube 2 beingenergized through choke 34 and coil 4|. Grid 4 of tube I, however,receives modulation energy directly in Fig. 3 by connection through highfrequency choke 27, resistor 51, which balances the impedance of thecircuit, and coupling condenser 23 to tap 32. A tuning condenser isshown at 55. The adjustments in Fig. 3 are similar to those aboveprescribed in reference to Fig. 1; condensers I2 and 63 which aredynamically in multiple in the output circuit are manipulated likecondensers 24 and 43 in Fig. 1, while the respective tuning condensers43 and 53 in the input are adjusted like condenser I2 in Fig. 1.Reference character 5i denotes a blocking condenser to isolate thepotential of the D. C. source from the antenna circuit.

Though not shown here, the system of my invention may also be-embodiedin a push pull arrangement. Inasmuch as the circuits illustrated inFigs. 1, 2 and 3 indicate that the two tubes I and 2 are dynamically inmultiple, even though different voltages may be effective therein, thepush pull arrangement would require four tubes or the equivalent inelectrodes. If two small tubes are used in push pull to furnish the baseportion of the wave, one large tube can be inductively or directlycoupled to my system to supply the cap portions. In my experiments Ihave also found that with certain degrees of input excitations I canproduce output currents where the two side bands are very pronouncedwhile the carrier is almost suppressed. I have also experimented withexcitations where one side band was'emphasized by means of suitableadjustment of condenser 66 of Fig. 3. Condenser 60 can be omitted ifdouble side band transmission is used. c

The high efficiency of the system of'my invention is evident from thefollowing experimen-. tal data. Theanode potential measured at tube lwas 800*volts, and the corresponding anode current at meter I was 75milliamperes; the negative grid bias was -110 volts and thecorresponding grid current flow at meter 36 was 100 microamperes. Fortube 2, the anode potential measured 800 volts, the anode current wasmilliamperes, and the negative grid bias measured 260 volts, whicheffectively blocked grid current. Carrier frequency power of'12 wattswas applied to the system by means of the'link circuit 40, and nomodulation'energy was employed at this time. The carrier frequency wasabout 33 megacycles/sec. The total plate input Letters Patent of theUnited States is as follows:

1. In combination, a continuously operable low level class C electrontube amplifier, an inter mittently operable high level class C electrontube amplifier, means for supplying carrier energy to said amplifiers insubstantially parallel paths, means for supplying modulation energy tosaid amplifiers for modulating said carrier energy in both saidamplifiers, means for selectively regulating the carrier and modulationenergy applied to said low level amplifier for delivering carrier energytherefrom at maximum efficiency sub-,

stantially at the level of unmodulated carrier energy and for limitingthe modulation therein substantially to the negative peaks of themodulation energy below the said level of unmodulated carrier energywhile the modulation in said high for both tubes was, therefore, 800 85=10- =68 watts. The effective output resistance was -'73 ohms and theefiective load current. 0.82 ampere at meter 38, giving anunmodulatedcarrier pow-- or output of 73x0.82 =49,watts. This yields an efficiencyof 4900/68=72%. Thereafter, modulation energy was applied for internalmodulation as has been described hereinbefore, the voltages on therespective anodes and grids being the same as for the unmodulatedcondition. For

100% modulation as verified by a cathode'oscillograph, the andoecurrentv I1 as measured with a direct current meter in the anode circuitof tube I dropped to only 15 milliamperes while the anode current oftube 2 rose to 100 milliamperes; the grid currents-were 700 microamperesin tube I and 3.5 milliamperes in tube 2. The link circuit 40 supplied12 watts unmodulatedcarrier power at 640 volts eifective, whilemodulation energy at 300 volts was effective for internal totalmodulation. Thesame load resistance of 73 ohms produced for an increasedmodulated output current of 1.05 amperes, or an output power of 80.3watts which, compared with the total anode input power of 93 watts,gives anode'efllciency as high as- 87%. It is to be noted that I cancompletely modulate 50 watts unmodulated carrier with only about 1 wattof modulation level amplifier is effected substantially under thepositive peaks of the modulation energy above the said level ofunmodulated carrier energy intermittently as such positive peaks occur,and an output circuit for combining the modulated carrier energy fromboth said amplifiers.

'2. In combination, a low level class C electron tube amplifier, a highlevel class C electron tube amplifier, separate circuits for supplyingcarrier energy to said amplifiers in substantially parallel relation, amodulation energy input circuit including parallel paths for supplyingmodulation energy to each of'said-ampliflers for modulating said carrierenergy in both said amplifiers, means connected with said low levelamplifier for regulating the carrier and modulation energy appliedthereto for delivering carrier energy therefrom at maximum efliciencysubstantially at the level of unmodulated carrier energy and forlimiting the modulation therein substantially to the negative peaks ofthe modulation energy below the said level of unmodulated carrier energywhile the modulation in said high level amplifier is effectedsubstantially under the positive peaks of the modulation energy abovethe said level of unmodulated carrier energy intermittently as suchpositive peaks occur, and an output circuit for combining the modulatedcarrier energy from both said amplifiers.

3. In combination, a low level class C electron tube amplifier, a highlevel class C electron tube power input, as compared with-the usualrequirement of watts modulation energy for completely modulating 50watts carrier energy in a high level class C amplifier.

The simplified control arrangement for adjusting the circuits formaximum efliciency is attributable to the dependence of current flow intube 2 upon the modulating voltage. Synchronism of the currents fromtubes l and 2 in the output is dependent, therefore, primarily on phaserelations in reference to the low frequency modulation and not the highfrequency carrier, and may accordingly be controlled with greater easeand stability. Phase relations at carrier frequency are of secondaryconcern, and inasmuch as purposeful phase displacement is avoided themaintenance of satisfactory phase relations in this respect isfacilitated.

amplifier, a carrier energy input circuit including parallel paths forsupplying carrier energy to each of said amplifiers, a modulation energyinput circuit including parallel paths for supplying modulation energyto each of said amplifiers for modulating said carrier energy in bothsaid amplifiers, means for selectively regulating the carrier andmodulation energy applied to said low level-amplifier for deliveringcarrier energy therefrom at maximum efliciency substantially at thelevel of unmodulated carrier energy and for limiting the modulationtherein substantially to the negative peaks of the modulation energybelow the saidlevel of unmodulated carrier energy while the modulationin said high level amplifier is effected substantially under thepositive peaks of the modulation energy above the said level ofunmodulated carrier energy inter- While I'have disclosed my invention incertain known embodiments thereof, I desire it understood that furthermodifications may be made therein, and that no limitations upon myinvention are intended except as may be imposed by the scope of theappended claims.

What I claim as new and desire to secure by mittently as such positivepeaks occur, and an output circuit for combining the modulated car'-rier energy from both said amplifiers.

4.The combination set forth in claim 2 ineluding tuning means in each ofsaid separate circuits for supplying carrier energy to said amplifiersand in said output circuit for adjusting the combination for operationat maximum eifh ciency.

5. The combination set forth in claim 3 wherein said amplifiers areconnected with parallel portions of said output circuit, and includingtuning means in said carrier energy input circuit and in each portion ofsaid output circuit for adjusting the combination for operation atmaximum efliciency.

6. In combination, a pair of non-linear electron tube amplifiers, meansfor supplying carrier energy to said amplifiers in substantiallyparallel paths, means for supplying modulation energy to said amplifiersfor modulating said carrier energy in both said amplifiers, an outputcircuit for combining the modulated carrier energy from both saidamplifiers, and means for selectively regulating the operation of eachof said amplifiers for limiting the modualtion in one or said amplifiersbelow the level of unmodulated carrierenergy therein substantially tothe negative peaks of the modulation energy and the modulation in theother of said amplifiers substantially to the positive peaks of themodulation energy intermittently as they occur.

7. In combination, a pair of non-linear electron tube amplifiers eachincluding an anode, a cathode and a grid electrode, means for energizingsaid electrodes including a source of grid bias potential individual toeach of said amplifiers, means for applying voltages of carrierfrequency in like phase to both said grid electrodes, means for applyingvoltages of modulation frequency to said grid electrodes for modulatingthe carrier energy in both said amplifiers, an output 1 circuit forcombining the modulated carrier energy from both said amplifiers, andmeans including said individual sources of grid bias potential forselectively regulating the operation of each of said amplifiers forlimiting the modulation in one of "said amplifiers below the level ofunmodulated carrier energy therein substantially to thenegative peaks ofthe modulation energy and the modulation in the other of said amplifierssubstantially to the positive peaks of the modulation energyintermittently as they occur.

8. The combination set forth in claim 7 wherein said sources of gridbias potential are adjusted for class C operation of said amplifiers,one at high level for positive peak modulation and the other at lowlevel, and including means connected with the amplifier operative at lowlevel, class C, for relieving the grid electrode therein of excessivevoltage at times when the applied voltage includes the positive peaks ofthe modulation energy.

9. In combination, an electron tube including cathode, grid and anodeelectrodes, a tuned circuit connected between said cathode and gridelectrodes, and means for biasing said grid electrode substantiallybeyond cut-oil? potential; a second electron tube including cathode,grid and anode electrodes, a second tuned circuit connected between thelast mentioned cathode and grid electrodes, and means tor biasing thelast said grid electrode substantially beyond cut-01f potential at alevel relatively higher than that of the bias potential on the firstsaid grid electrode; means for supplying energy of carrier frequency toboth said tuned circuits, and means for applying modulation energysimultaneously to both said grid electrodes for modulatingsaid energy ofcarrier frequency in both said electron tubes, modulation in the firstsaid electron tube being efiected substantially under the negative peaksof said modulation energy below the level of 'unmodulated carrier energyand that in the said second electron tube substantially under thepositive peaks of said modulation energy above the level of unmodulatedcarrier energy, by virtue of the difierent grid bias potentials; and anoutput circuit for combining the modulated carrier energy delivered fromboth said electron tubes.

10. The combination set forth in claim 12 including tuning means in saidoutput circuit, the aforesaid tuned circuits being' individuallyadjusted for maximum efilciency of operation in said electron tubes andsaid output circuit being tuned by said tuning means for resonance withboth said tuned circuits.

'11. The combination set forth in claim 9 including a resistance and ablocking condenser connected in series between the cathode and gridelectrodes in the first said electron tube;' said resistor having apotential drop thereacross, resulting from carrier and modulation energytherein, efiective to suppress modulation in the first'said electrontube substantially under the positive peaks of said modulation energy.

12. In combination, an electron tube including cathode, grid and anodeelectrodes, means for biasing said grid electrode substantially beyondcut-oi! potential, and a tuned output circuit con-,

nected between said cathode and anode electrodes; a second electron tubeincluding cathode, grid and anode electrodes, means for biasing the lastsaid grid electrode substantially beyond cutofif potential at a levelrelatively higher than that of the bias potential on the first said gridelectrode, and a tuned output circuit connected between the cathode andanode electrodes in the said second electron tube; an input circuit forapplying energy of carrier frequency to both said grid electrodes, andmeans for applying modulation energy simultaneously to both said gridelectrodes'for modulating said energy of carrier frequency in both saidelectron tubes, modulation in the first said electron tube beingeffected substantially under th negative peaks of said modulation energybelow the level of unmodulated carrier energy and that in the saidsecond electron tube substantially under the positive peaks of saidmodulation energy above the level of unmodulated. carrier energy, byvirtue of the different grid bias potentials; said tuned output circuitshaving-portions in common for combining the modulated carrier energydelivered from both said electron tubes.

13. The combination set forth in claim 12 including tuning means in saidinput circuit; the said tuned output circuits being individuallyadjusted for maximum emciency of operation in said electron tubes andsaid input circuit being tuned by said tuning means for resonance withboth said tuned output-circuits.

14. The combination set forth in claim 12 in-' cluding a resistance anda blocking condenser ROBERT E. TAYLOR.

